A helical antenna array generally comprises a series of helical antenna elements, each one of which comprising a conductor, such as a wire, tape, moulded conductor, stamped conductor, extrusion, or printed circuit, having a nominally helical geometry that, when energized, generates a circularly or substantially circularly polarized beam. In some realisations the helices may have more than one winding, where the windings may have the same or different pitches and the same or different starting positions. To ensure structural integrity, the helical winding is usually supported by a dielectric former consisting of a cylinder or the like, and as such has a substantially circular helix cross-section. Helical antenna arrays may further comprise a ground plane, which provides a signal return or ground connection for the RF source of the antenna elements, and can further reflect that part of the electromagnetic wave generated by the antenna elements that propagates in the rearward direction, i.e. the ground plane effectively re-directs this radiation forwards. The live terminal of the RF source, on the other hand, connects to the starting point of the antenna's helical winding, which in some cases lies proximal to or almost immediately above the ground plane. Thus, the ground plane may provide circuit continuity for the input transmission line, usually a coaxial cable, which excites the antenna. For example, the center conductor of the coaxial line connects to the end of the helical winding, whereas the outer conductor of the coaxial line connects to the ground plane. The ground plane may have a planar surface, or alternatively, may consist of a cup, as shown in U.S. Pat. No. 6,664,938. In some realisations there may be no ground plane with the wave being launched either between adjacent windings or at a point along one or more windings.
The performance of relatively small helical antenna elements can be characterized, at least in part, by a gain parameter, which usually ranges from 5 to 12 dBIc. While in some cases, higher gain levels in excess of 12 dBIc can be achieved by using longer helices, significantly large length increments are often required to achieve relatively small gain increments. Therefore, a helix antenna is generally considered to be more efficient in terms of gain achieved as related to structural volume, when it is relatively short. For many purposes, a more expedient solution to achieving higher gains is to assemble an array of moderately sized helices.
In some applications, such as those shown in US Patent Application Publication No. 2008/0012787, a helical antenna element may have a conical shape, where the winding diameter at the feed end of the winding may be greater than the diameter at the radiating end. Conical helix structures may be advantageous when a helix antenna is to be operated over a wide frequency band. In other applications, such as the ones shown in U.S. Pat. No. 6,172,655 and US Patent Application Publication No. 2004/0135732, helices are wound about formers of varying cross-section diameters, increasing linearly toward a central maximum, and reducing linearly thereafter. Antenna elements of this type are commonly known in the art to provide for increased broadband performance. These examples may further comprise varying helix winding densities, wherein a winding has smaller pitches at the feed end and larger pitches at the radiating end.
As will be appreciated by the person of ordinary skill in the art, a helix is generally excited by connecting the lower extremity of its winding to an RF source. An electromagnetic wave then travels around the winding. This wave ultimately launches radiated fields when it arrives at the top the radiating or terminal end of the winding. A major portion of the radiated fields then propagates forwards, following a direction that is dictated predominantly by the phase distribution of the wave along the helix winding. In the design of high gain, fixed beam arrays, it is generally desirable to design the individual helices for maximum gain along the axis of the helix winding.
Many factors may contribute to the reduction of the gain of a helical antenna: the termination of the antenna, if open-circuited, carries no current; the dielectric material of the support structure may introduce dissipative losses and stored energy with related mismatch losses; mutual coupling between adjacent helices can broaden the beam; the axial design of conventional helices makes inefficient use of the volume within which the antenna may be rotated; and the high launching impedance resulting from small winding diameters can result in an inferior matching structure.
When several helices are assembled together so as to form an array, electromagnetic couplings may occur between neighbouring helices. Conventional excitation of the array with uniform helix orientations exacerbates this problem by maximising the coupling between the elements. One impact of the coupling is to progressively pull the patterns of the individual elements towards the centre of the array. The individual elements of the array then radiate in different directions, thereby reducing the gain of the array. Additionally, the coupling narrows the impedance bandwidth, and may increase mismatch loss. For example, in a four-element array comprising non-helical elements, a power gain of roughly 5 dB can be achieved using the array, over the gain of a single element. Given the electromagnetic couplings between helix elements, however, a four-element helix array is more likely to have a power gain of only 4 dB higher than that of a single helix element.
U.S. Pat. No. 5,874,927 provides one approach to improving the performance of a helical antenna array by tilting the otherwise linear helical antenna elements away from one another, whereby such tilting is reported to broaden the effective aperture of the array. This approach, while providing some advantages over parallel implementations, also has the effect of increasing the overall sweeping radius of the array, which, in some embodiments where spatial limitations are of crucial importance, can limit the applicability of such design.
For example, helical antenna arrays are commonly used for satellite communications in aircrafts or the like. Examples of satellite communications may include, but are not limited to, airborne and/or ground based communications for receiving weather reports and/or air traffic control information, or for communicating status and emergency messages, to name a few. Furthermore, such satellite communication systems may also be useful in providing such services as telephone communications, Internet services, and/or other forms of data exchange to the aircraft passengers. In the context of aircraft communications, helical antenna arrays are commonly mounted at the tail section of an airplane or the like, which tends to be very narrow and may limit the size of the antenna array that can be deployed. Consequently, a person of ordinary skill in the art would appreciate that the installation and operation of a helical antenna array for aircraft communications may impose certain operational and structural limitations to the type of antenna suitable for such applications.
Furthermore, as aircraft communication systems generally relay communications via a link from the aircraft to a communications satellite, which communications are then relayed to grounded resources via a separate link, and since such systems are generally expected to function independently of the position of the aircraft around the globe, the associated aircraft communications antenna should generally be capable of pointing its radiation towards a selected satellite at all times. Accordingly, the antenna beam should be steered by appropriate means depending on the local latitude and longitude of the aircraft, the attitude of the aircraft, and the heading of the aircraft. In some applications, an electronic steering method is used to reduce the number of mechanically moving or turning parts of the antenna structure. However, such steering methods generally are not applied to single helix implementations. Rather, mechanical steering methods may be used alone or in combination with electronic steering. As noted above, however, the aircraft may impose certain limitations relating to the available spaces within which the antenna can be installed and operated (i.e. steered). These limitations place very demanding constraints on the size of the antenna assembly, and the scan envelope volume that the antenna assembly requires. For instance, in order to mechanically steer the antenna within the tail section of the aircraft to scan a desired coverage area, spatial limitations should generally be respected irrespective of antenna orientation, namely, the antenna should operate freely within a scan radius or volume as prescribed by a radome covering a top portion of the aircraft tail section and the antenna in operation. Similarly, radomes on top of trucks, trains, ships, fuselages and other vehicles are compact and may limit the sweeping volume of the antenna installed.
Accordingly, solutions as provided by U.S. Pat. No. 5,874,927, while providing some operational advantages over standard arrays, may be of limited suitability in the above context where spatial limitation applies, or where an increase to an array sweep radius cannot generally be accommodated in standard installations.
Therefore there is a need for a new helical antenna element and array thereof that overcomes some of the drawbacks of known antenna arrays, or that provides the public with a useful alternative.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.